In the United States alone, approximately 25% of patients in need of organ transplants die while waiting for a suitable donor. The current demands for transplant organs and tissues are far outpacing the supply, and all manner of projections indicate that this gap will continue to widen. Cell transplantation was proposed as an alternative treatment to whole organ transplantation for failing or malfunctioning organs. For the creation of an autologous implant, donor tissue is harvested and dissociated into individual cells, and the cells are attached and cultured onto a proper substrate that is ultimately implanted at the desired site of the functioning tissue. Because many isolated cell populations can be expanded in-vitro using cell culture techniques, only a very small number of donor cells may be necessary to prepare such implants. However, it is believed that isolated cells cannot form new tissues, independently. Most primary organ cells are believed to be anchorage-dependent and require specific environments that very often include the presence of a supporting material to act as a template for growth. The success of any cell transplantation therapy therefore relies on the development of suitable substrates for both in-vitro and in-vivo tissue culture. Currently, these substrates, mainly in the form of tissue engineering scaffolds, prove less than ideal for applications, not only because they lack mechanical strength, but they also suffer from a lack of interconnection channels (Shoufeng Yang, Kah-Fai Leong, Zhaohui Du, and Chee-Kai Chua. The Design of Scaffolds for Use in Tissue Engineering. Part I. Traditional Factors. Tissue Engineering Vol. 7 No. 6, 2001).
Tissue engineering applications or even in 3D cell cultures, the biological cross talk between cells and the scaffold is controlled by the material properties and scaffold characteristics. In order to induce cell adhesion, proliferation, and activation, materials used for the fabrication of scaffolds must possess requirements such as intrinsic biocompatibility and proper chemistry to induce molecular bio-recognition from cells. Materials, scaffold mechanical properties and degradation kinetics should be adapted to the specific tissue engineering application to guarantee the required mechanical functions and to accomplish the rate of the new-tissue formation. For scaffolds, pore distribution, exposed surface area, and porosity play a major role, whose amount and distribution influence the penetration and the rate of penetration of cells within the scaffold volume, the architecture of the produced extracellular matrix, and for tissue engineering applications, the final effectiveness of the regenerative process. Depending on the fabrication process, scaffolds with different architecture can be obtained, with random or tailored pore distribution. In the recent years, rapid prototyping computer-controlled techniques have been applied to the fabrication of scaffolds with ordered geometry (Carletti E, Motta A, and Migliaresi C. Scaffolds for tissue engineering and 3D cell culture. Methods Mol Biol. 2011; 695:17-39).
Since the publication of Cajal's pioneering studies, it has been clear that neurons from the central nervous system (CNS) regenerate poorly, in contrast to those from the peripheral nervous system (PNS). Throughout adult life, olfactory sensory neurons are continuously replenished from progenitor cells of the olfactory neuroepithelium. Furthermore, unlike other peripheral nervous system (PNS) neurons, these neurons extend axons that reach their final targets in the central nervous system (CNS)-situated olfactory bulb (OB) (Farbman A I. Olfactory neurogenesis: genetic or environmental controls? Trends Neurosci. 13:362-5. 1990; Doucette R. Glial cells in the nerve fiber layer of the main olfactory bulb of embryonic and adult mammals. Microsc Res Tech. 24:113-30. 1993).
Olfactory ensheathing cells (OECs) are a unique glial cell type that resides in the olfactory bulb and in the olfactory mucosa (Richter M, Westendorf K, Roskams A J. Culturing olfactory ensheathing cells from the mouse olfactory epithelium. Methods Mol Biol. 438:95-102. 2008; Jani H R, Raisman G. Ensheathing cell cultures from the olfactory bulb and mucosa. Glia. 47:130-7. 2004). OECs envelop olfactory sensory axons along their way to the target neurons in the olfactory bulb. Thus, OECs have drawn much attention with respect to CNS axonal regeneration (Moreno-Flores M T, Diaz-Nido J, Wandosell F, Avila J. Olfactory Ensheathing Glia: Drivers of Axonal Regeneration in the Central Nervous System? J Biomed Biotechnol. 2:37-43. 2002) and have been proposed to facilitate this process in the injured CNS (Ramon-Cueto A, Nieto-Sampedro M. Regeneration into the spinal cord of transected dorsal root axons is promoted by ensheathing glia transplants. Exp Neurol. 127:232-44. 1994). Indeed, the regenerative capacity of these cells in the injured spinal cord and their ability to remyelinate injured spinal cord axons has been confirmed in several studies (Imaizumi T, Lankford K L, Waxman S G, Greer C A, Kocsis J D. Transplanted olfactory ensheathing cells remyelinate and enhance axonal conduction in the demyelinated dorsal columns of the rat spinal cord. J Neurosci. 18:6176-85. 1998). OECs have been suggested to support axon growth in the injured CNS via expression of growth factors, such as NGF, NT4/5, NT3, and BDNF (Lipson A C, Widenfalk J, Lindqvist E, Ebendal T, Olson L. Neurotrophic properties of olfactory ensheathing glia. Exp Neurol. 180:167-71. 2003; Woodhall E, West A K, Chuah M I. Cultured olfactory ensheathing cells express nerve growth factor, brain-derived neurotrophic factor, glia cell line-derived neurotrophic factor and their receptors. Brain Res Mol Brain Res. 88:203-13. 2001).